Build Element Call as a component a host can import

Adds component/index.tsx as a fourth build target: <ElementCall client
roomId /> and an initializeElementCall to await once beforehand. It gives
Element Call everything it would otherwise take from the page it is on —
the parameters, the host bridge, media devices, translations, a container
to confine itself to — and hands it the host's client rather than finding
one of its own.

React, the Matrix SDK and LiveKit stay external, since the host has them
and a second copy of any would not merely be wasteful: React would hold
two sets of hooks and the client would run two sync loops. Every subpath
has to be listed by name, because the pattern and callback forms of
rollupOptions.external are silently ignored here — a lesson worth the
comment that records it.

Element Call's own navigation runs in a MemoryRouter, so being embedded
cannot disturb the host's URL. ClientContext and GroupCallView both
navigate, so some router has to be present.

The bundle is not yet a reasonable size: library mode base64-inlines
assets referenced through import.meta.url, so MediaPipe's vision runtime
lands in it whole. Left for its own change, since the fix — loading the
background blur transformer lazily — is worth doing for the standalone app
too.

vite-component.config.ts

@@ -0,0 +1,65 @@
+/*
+Copyright 2026 Element Creations Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE in the repository root for full details.
+*/
+
+import { defineConfig } from "vite";
+
+import { vitePluginsConfig } from "./vite.config";
+
+// Config for Element Call as a React component, to be imported by an
+// application embedding it rather than served as a page of its own.
+//
+// Deliberately not built on top of the full app's config, which exists to
+// produce a page and brings an HTML entry point along with it.
+export default defineConfig(({ mode }) => ({
+  ...vitePluginsConfig({ mode }),
+  build: {
+    minify: mode === "production",
+    sourcemap: true,
+    // One stylesheet rather than one per chunk, so a host has a single file to
+    // include
+    cssCodeSplit: false,
+    lib: {
+      formats: ["es" as const],
+      entry: "./component/index.tsx",
+      fileName: "element-call",
+    },
+    rollupOptions: {
+      // The host already has these, and a second copy of any of them does not
+      // merely bloat the bundle: React would hold two sets of hooks, and the
+      // Matrix client would run two sync loops.
+      //
+      // Every subpath has to be named. Element Call reaches most of the Matrix
+      // SDK as `matrix-js-sdk/lib/…`, and a bare "matrix-js-sdk" would not
+      // catch those — while the pattern and callback forms of this option are
+      // silently ignored by the bundler, so they cannot be used to cover them.
+      // `pnpm lint:externals` fails if an import appears that is not listed.
+      external: [
+        "react",
+        "react/jsx-runtime",
+        "react-dom",
+        "react-dom/client",
+        "livekit-client",
+        "matrix-js-sdk",
+        "matrix-js-sdk/lib/client",
+        "matrix-js-sdk/lib/crypto-api",
+        "matrix-js-sdk/lib/logger",
+        "matrix-js-sdk/lib/matrix",
+        "matrix-js-sdk/lib/matrixrtc",
+        "matrix-js-sdk/lib/matrixrtc/EncryptionManager",
+        "matrix-js-sdk/lib/matrixrtc/IKeyTransport",
+        "matrix-js-sdk/lib/matrixrtc/IMembershipManager",
+        "matrix-js-sdk/lib/models/relations-container",
+        "matrix-js-sdk/lib/models/room",
+        "matrix-js-sdk/lib/models/typed-event-emitter",
+        "matrix-js-sdk/lib/randomstring",
+        "matrix-js-sdk/lib/sync",
+        "matrix-js-sdk/lib/types",
+        "matrix-js-sdk/lib/utils",
+      ],
+    },
+  },
+}));